Self-commutated converters are used at present in a number of applications, for example in static converters for motor drives, switched supply devices, UPS systems, etc. Normally, in such applications, an individual power semiconductor is utilized for building up the occurring relatively high voltages. In certain applications, however, it is desired to switch off or on such a high voltage and this cannot be managed with a single power semiconductor. Power semiconductors then have to be capable of being series-connected with a good voltage division. As examples of applications in which it may be of interest to series-connect power components, it may be mentioned converters for high-voltage motors, traction motors or high-voltage generators. Further, there are applications within distribution and transmission of electric power wherein very high voltages (typically 20-500 kV) are switched on or off. High-voltage converters are used here, among other things as controlled rectifiers and inverters for transmission of high-voltage direct current (HVDC). It is considered that self-commutated converters of a voltage-source type will be used to an increasing extent in transmission and distribution applications. Besides in HVDC there may be mentioned, as examples, converters for reactive power compensation, phase angle control, improvement of the network quality (e.g. compensation of harmonics induced by non-linear loads), for transmission of power between the phases during a phase error, etc. In more general terms, these converters may be used to control power flows, provide better availability and higher network quality in existing AC systems.
In self-commutated converters there are used gate turn-off power semiconductors of GTO thyristor type or controllable power semiconductors of transistor type, for example IGBT, MOSFET, bipolar transistors (BJT), or Darlington transistors. In the higher power range, especially GTO thyristors and IGBT transistors, respectively, are at present the most interesting components. IGBT transistors are predicted to handle increasingly larger powers and successively to replace or supplement GTO thyristors in converters intended for high powers and high voltages.
In series connection of power semiconductors, which are in the form of components as IGBT or GTO, however, it is difficult to obtain a uniform voltage distribution between the components. This applies especially to turn-off of the current, but problems with voltage division may also arise during blocking and turn-on. For the IGBT case, a good voltage division is also required during a possible short circuit of a DC intermediate-link capacitor since the IGBT builds up voltage and limits the current. Taken together, these have contributed to the fact that the number of commercial applications using series-connected power components of the GTO or IGBT type is still very small.
The problems in connection with turn-off arise from the fact that the series-connected power components tend not to build up voltage completely synchronously. Certain components tend to turn off earlier than others, which easily causes overvoltages to arise for those components which turn off early in a valve, by valve being meant the chain of series-connected components.
Similar problems may also arise during turn-on of the power components. Certain components turn on later than others, which may lead to a brief overvoltage across the components which turn on later. However, the problems are normally smaller in the case of turn-on of the valve than in the case of its turn-off.
Further, during blocking of the valve, certain components may have a lower leakage current than others, which results in higher voltage across these components in a valve chain. However, this problem is normally relatively simple to over-come with a resistive voltage divider arranged in parallel with each power component in the chain.
During short-circuiting of the dc intermediate link capacitor, in the IGBT case, component variations will result in certain IGBTs starting to build up voltage at a lower current than others, which leads to an uneven division of the voltage and problems with overvoltages on certain components.
In forced-commutated converters of a voltage-source type, there is normally also an antiparallel diode in parallel with the turn-off power semiconductor (e.g. IGBT or GTO). This diode carries current in the opposite direction to the direction in which the turn-off power semiconductor carries current in the chain of series-connected power semiconductors (FIG. 1). When the mentioned diode turns off and builds up voltage, problems with voltage division may arise also in this case. The mentioned antiparallel diode constitutes a power semiconductor which must be protected against overvoltages caused by an uneven division of the voltage across the series-connected diodes in the chain. In this document, the concept of series-connected power semiconductors also comprises power semiconductors connected in parallel across the controllable and turn-off power semiconductors. Normally, however, the voltage division during turn-off of the power semiconductors (GTO, IGBT) is the case which is most difficult to handle.
In GTO applications (not series connection of power semiconductors), it is common to apply a so-called RCD snubber in parallel with the GTO. FIG. 2 shows an example of an RCD snubber, where DU designates a drive unit, T1 a GTO, D1 an antiparallel diode, and SN comprises the snubber. Such a snubber is intended to limit the speed of the voltage growth across the GTO, which increases the current handling capacity of the GTO and reduces the turn-off losses thereof. An RCD snubber also improves the voltage division during series connection of such components. To obtain a good voltage division, however, a capacitor included in the snubber must be large, which normally means that the snubber losses become too large to handle. SE patent 8801140-8 discloses a method in which, by introducing individual delays in the control-pulse generation, the voltage division may be improved without need to choose a very large snubber capacitor. The method is based on the temperature of an overvoltage protection device, in this case a varistor, being measured individually for each thyristor and on a signal, based on the measurement, being used to achieve an individual delay of the turn-off time of the thyristor.
In IGBT applications (not series connection), it is common to use simpler types of snubber devices or to exclude the snubber completely. The reason for this is that the IGBT can normally handle a faster voltage growth during turn-off than the GTO. However, it is often suitable to have some type of overvoltage protection device to limit the voltage peak which always occurs during commutation of a current in a circuit with a certain inductance (see, e.g. "Protection of IGBT Modules in Inverter Circuits", EPE Journal, Vol. 1, No. 1, July 1991). One example of such a protection device of the type mentioned, which is effective with respect to overvoltages, is a clamping capacitor which feeds back its charge to a dc intermediate link (FIG. 3). This type of snubber does not limit the voltage build-up across the IGBT until it has reached the value to which the clamping capacitor is charged. This results in the losses in the snubber to becoming very low while at the same time overvoltages in connection with turn-off are effectively limited.
During series connection of IGBTs, however, some type of overvoltage protection is needed, which prevents individual components, which turn off earlier than others, from building up too high a voltage. A natural choice for achieving a passive overvoltage protection device is then to use the same type of snubber as for GTO applications, namely, an RCD snubber. However, the losses in such an RCD snubber easily become relatively great since it is normally difficult to cause all the series-connected IGBTs to turn off at exactly the same time.
An overvoltage protection device may also be active. By an active overvoltage protection device it is meant that the protection device influences the control of the component such that it tends to increase its conductivity again if the voltage across the component rises too high. A natural choice to achieve an active overvoltage protection device for an IGBT is simply to connect a Zener diode in series with a diode between the collector and the gate (FIG. 4). This method is described, for example, in "Switching Voltage Transient Protection Schemes for High Current IGBT Modules", IEEE 1994. The disadvantages of this concept, according to the authors of the article, are that the stray capacitances in the protective components may influence the turn-off process negatively and that the protective method increases the turn-off losses during turn-off of normal currents.
Active protection devices also exist in other forms, for example, an active snubber. An active snubber comprises, for example, a capacitor and a resistance between the collector and the gate on an IGBT (FIG. 5) and the object thereof is to limit the voltage derivative during turn-on and turn-off, respectively. This technique is described, for example, in "An Investigation of the Drive Circuit Requirements for the Power Insulated Gate Bipolar Transistor (IGBT)", IEEE Trans on Power Electronics, Vol. 6, No. 2, April 1991.